Low-pass filter system



Patented June 14, 1949 'time LOW-PASS FILTER SYSTEM Gordon L. Fredendall, Feasterville, to Radio Corporation of America,

of Delaware Pa., assignor a corporation Application November 29, 1943, Serial No. 512,115

(Cl. ri-44) 13 Claims.

This invention relates generally to wave transmission networks and more particularly to adjustable low-pass networks employing a plurality of fixed lter circuits.

l Heretofore, adjustable low-pass filters having a continuously variable cut-off frequency have included a plurality of individually adjustable reactive filter components. Adjustable reactive elements, suitable for low-pass networks, are costly and cumbersome. It also is costly and inconvenient to gang a large number of adjustable reactive elements for convenient unitary adjust-V ment of the cut-off frequency.

The instant invention contemplates broadly converting the currents of one frequency into currents of a higher frequency. The higher frequency currents are applied to a low-pass network employing only two fixed reactive filter networks in combination with two fixed modulators and two adjustable carrier frequency sources such as, for example, thermionic tube oscillators. The two fixed filter networks have frequency band pass characteristics at frequencies related to but appreciably higher than the desired low-pass cutoff frequency. The conversion to higher frequencies reduces the physical size and cost of the filter components.

The signal frequency currents which are to be attenuated are applied to a first carrier frequency from one of the oscillator circuits to provide two equal side bands. The two side bands and the rst carrier frequency are applied to the first filter network to attenuate only the upper side band to the desired low-pass band width. The unequally attenuated side bands and the first carrier are then applied to the second carrier frequency which is selected to be substantially higher than the first carrier frequency. The second fixed filter network has a pass band which attenuates the lower part of the upper band side band to a value substantially equal to the upper part of the upper band side band, thereby providinga high frequency carrier having two side bands each having widths equivalent to the desired low-pass hand. The high frequency modulated carrier is applied to a demodulator or envelope detector to derive the low-pass frequency band desired.

, Among the objects of the invention are to provide an improved method of and means for transmitting a predetermined low-pass frequency band. Another object of the invention is to provide an improved low-pass transmission network employing relatively high frequency band-pass filter netlator will include a works. An additional object of the invention is to provide an improved adjustable low-pass transmission network employing a plurality of fixed band-pass filter networks.

A further object of the invention is to provide an improved adjustable low-pass transmission network employing in combination a plurality of xed filter networks, a plurality of adjustable carrier frequency sources and means for modulating the signal frequencies with the carrier frequency sources to provide the desired low-pass frequency transmission band.

The invention will be further described by reference to the accompanying drawing of which Figure 1 is a schematic circuit block diagram of a preferred embodiment, Figure 2 is a family of graphs indicating the operation of the circuit of Figure 1, Figure 3 is a schematic circuit diagram of a typical fixed band-pass filter network forming one of the elements of the circuit of Figure 1 and Figure 4 is a graph illustrative of the frequency band-pass characteristics of filter networks of the type described in Figure 3. Similar reference characters are applied to similar elements throughout the drawing.

Referring to Figure l of the drawing, a source of signals occupying a frequency band from zero to a frequency f1 is applied to the input circuit of a first thermionic tube modulator 3. A source of carrier frequency currents such as, for example, a thermionic tube oscillator 5 also is connected to the input circuit of the first modulator 3 to provide modulated carrier currents in the output circuit of the modulator 3. if the frequency ,fcl of the first carrier frequency oscillator 5 is appreciably higher than the highest signal frequency f1, the currents derived from the first moducarrier and sidebands covering a frequency spectrum from (fcl-fD to (fcl-i-fl).

This frequency spectrum is applied to the input of a fixed band-pass filter network having a pass band (fc1-,1-A) to (icl-H2). It will be seen that the lower sideband (fel-f1) will not be attenuated by the band-pass filter l while the upper sideband (fel-H1) will be attenuated to a value (icl-H2) The currents derived from the first band-pass filter 'l are applied to the input circuit of a second thermionic tube modulator 9. The output of a second carrier frequency source such as, for example, a thermionic tube oscillator ll having a carrier frequency fc2, is also applied to the input of the second modulator Si. The modulated carrier derived from the second modulator 9 will include carrier and sidebands covering the spectra (fcZ-fcl) f2 to (fc2-fc1) +f1 and (fc2+fc1) -fl to (fc2-{-fc1)f-f2. These two sidebands and the second carrier frequency fc2 are applied to the input of a second bandpass lterl network having a pass band covering the spec; trum fc2+fc41-f2 to f2+f1+f2+nf It will be seen that the second band-pass filter will at` tenuate the lower portion of the upper sideband to a band width equivalent to f2, and will not affect the transmission of the upper portion of the upper sideband which already has a band width of f2. Therefore, the output of the second bandpass filter I3 will comprise'ff'requencies covering4 the spectrum (fc2-l-fc1) -f2 to (fc2-l-fc1) -l-f2 or, in other words, a carrier (jez-Hol) having equal upper and lower sidebands f2;

The output of the second band-pass filter I3. is applied to a demodulator I5 such as, for example, a diode thermionic. tubi-' ,eto,derive` currents covering; thefrequencygspectrum from zero to f2, which isthe, desired'lo'w frequency spectrum4 from z'er'o'to f2, which'is the diesiredlow frequency pass baud yIf`it isA desired to lc lerivea`v` pass band from zero to a.frequencyf3.;fwh'ee f3 f2: f1, it merely is necessary"to'increase'the frequency of the rst Crlier frquencv'fci to ,ayalue (fc1-l-f2*f3) and s1' ,n1'i`J.lta Iitliya5indifaledbyV the dash line 33 of Fig; 1, tofdecfraserthesecondcarrier frequency f'c2 toA`[fc2-'-2f(f2'-f3 l. Similarly, if it is desired toincrease` thepass band, the .rst carrier fcl `will lie/decreased in'frequencyand the second carrier f`c 2`,`w il1flbe increasedgin 'frequency in the same manner 'asdescribed heretofore.

Inrtigurez. eraphaindieates the. Original, Signal frequency,,bandy '0tol `)"land the desired low-pass bandlj to f2. The rstcarrierfrequency isindicatdb'y, thedasljifline fol andthe output currents derivedlfrom therst modulator are indicated by thefrequeney spptrunr (fai-fai) to fc1+f1 Graph'bindicates' the parie-pass characteristics of the Afirst and second 'band-pass lters.- Graph c'indicat'es'th'e output-currents derived from the rst lter networkfshowing theattenuation of the upper 4sideband v(fcl-l-Jl) 5to a value f2.

Graph d'illustrates thesecond carrier frequency, indicatedfby thedash line fc2, and the upper and lower sidebands provided-byA the modulation of `the second carrier bywthe modulated. rst carrier..V Graph fe .illustratestheattenuation: of `the lower portion .offthe `upper..sideband and t -e..lack. of attenuation; olii-the .upper -portionmof the upper sidebandwhereby two equall sidebands having a band widthfZ and-acarried (fc2-l-fcl) are .obtained .frnmmhesecondflten network.L As; explained heretofore., the `currents illustrated, by the graph. e are applied :to ,the .demodulatorl5 to, derive the desired low frequencypass band.

The fixed band-pass lter networks I,- I3 -may beof any conventionaligtype known `inf the art which. Willygprevide ftheLdeSirei-.fliequeeey bendf, passrspectra and permissible phase. distortion. A4 typical bandeplass filter Vneetvvork Iis illustrated schematically. inFigure andv includes a first resonant. `circuit I 1 -Y comprising a .parallel-con,- nected .inductor.,`V I 9,.,an1l cai'oacitor 2L. coupled throughcou'pling .capacitors 4v23, -25 to a second resonant .circitfll comprising a `parallel-con,-v nected in'ductor 29and ,capacitor 3|., Theparallel. resonant circuits 4| 1, .Z'Lmay bevtuned to the same orislighltly diierentrespnant frequencies, and, if desired, the.,inclluctorsl ,I9, 29, may be inductively aswell Aas icapacitively coupled. Likewise, the

coupling between'th'e resonant circuits I'I, 21 may" 75 be entirelyy inductive and the coupling capacitors 23, 25 omitted.

Figure 4 illustrates a typical response curve for a band-pass filter of the type described in Figure 3 wherein substantially uniform lattenuation is provided at all frequencies except those within a predetermined frequency band wherein extremely low attenuation; i. e., good-transmission is provided for the desired 'applied signal currents.

It should be understood that any other known types of band-pass lte'r networks may be substituted for the type described in Figure 3J and that phase distortion of the signal may be minimized to. any desired extent by applying Bodes theory tothe design of linear phase shift filters in each of the band-pass filter networks. The advantages ofthe low-pass network thus described areV particularly outstanding if an adjustable network providing low phase distortion of the signal is desired. Adjustable low-pass filters, having a variable cut=oit frequency, designed according tojBodes theory; would require'the gauging of numerous variable capacitorsand inductors;

Thus the invention described'compris'es van im# proved means for and method of securing low band-pass -1 transmission of* signal" frequency' cur'i rentsby' conversions of* low frequencies l to higher frequencies-and-byemployingrelativelyhigh"fre= quencyvband-'pass filters infcombination' withV conventionalL th'ermionicf tubemodulators and adjustable'fthermionic tube oscillatorsl After the required frequency-conversions andattenuations,v the original signal frequencies; lessfthe'undesired frequencies, are obtainedly by' demodl'lltion.` In the; described vsystem `relatively simple frequency ChangeSbring; about @thea desired tb'and-'pass' lcharacteristics instead'4 of :changing Ythe freactive' char'- acteristic of band-pass filters; which reactive changes yarezusually accompaniedK withuundesired 40 phase/shifts.

I `claim ,as-my: invention:

1'.4 Apparatuszfor providinglombandpass transmission` vofgfsgnal, `frequency fcurrents z including means,... forpderiving; from'` said;A signal .1 currents second-' currents having aa;v carrier andstwol side bands; one` Ofatsaidfsidebands. having .the desired. frequency band width, means for derivingifrom; said-j secondi currents g thirdncurrents.V having a, second,-gcarrier -andfzztwo equals-second 'sidebands each having the zdesiredifrequencyzband:.widthi and detectionrand tilterrmeans forlde'modulating; said third currentsz'to denve-Afourtl'ilcurrents hav#- irigsubstantiallyonlyrfrequencieszfwithimsaid 'dei sired lowpassbandn.

2:, Apparatusonprovidingfadi ustableiow band pass transmissionnoffsignalj.frequency? currents including .fmeans .Ii-fori.'- derivingqfrom i: said 1 signal currents-second .currents having ,a'fcarrier l and two lsidebandS.-.-,onepf saidsidebands havingthe desired f ,frequency band width; .means for deriv.- ingfr.omfsaid secondcurrentsthird currents hav-1 ingya ,second carrierand two=equali second-.sida bandseach -hayingathe desiredafrequency :bandwidth,fme'ans for,.adj ustinglfthe- .carriei` frequency upon A which each of -said:sidebandsdepend, and

detection and -Ilterz-,means fondemodulating sa-id' third currents... td derive fourth.. currentsehaving. substantially only frequencies Withinfsaiddesired 1ow.pass,.band.

3..,A low..ban d ;pass,.network forma .source fof. signal .l frequency. I currents.. including. ..a pair-.0L modulators, a pairofcarrier current sources hav ingidierent carrienfrequencies, .a pair. of. filter.` networkseach capable.of,..transmitting.,a Iband` of frequencies including a different one of said'M having substantially only e carrier frequencies-means including `one of said modulators responsive to said signal source and to one of said carrier current sources for deriving from one of said filter networks said one carrier and two sidebands, one of said sidebands having the desired ylow pass band width, means including said other modulator and said other filter network responsive to said frequencies derived from said one lter and said other carrier sourcefor yderiving currents of said other carrier and two equal second sidebands each having said desired low pass band width, and means for detectingsaid last mentioned equal sidebands to derive currents frequencies within said desired low pass band. f

4. An adjustable low band pass network for a source of signal frequency currents including a pair of modulators, a pair of carrier current sources having adjustabie different carrier frequencies, a pair of filter networks each capable of transmitting a band of frequencies including a different one of said carrier frequencies, means including one of said modulators responsive to said signal source and to one of said carrier current sources for deriving from one of said filter networks said one carrier and two sidebands, one of said sidebands having the desired low pass band width, means including said other modulator and said other filter network responsive to said frequencies derived from said one filter and said other carrier source for deriving currents of said other carrier and two equal second sidebands each having said desired low pass band width, means for adjusting the frequencies of said carrier current sources, and means for detecting said last mentioned equal sidebands to derive currents having. substantially only Afrefquencies within said desired low pass band.

5. An adjustable. low band pass network for a source of signal frequency currents including a pair of modulators, a pair of carrier current sources having adjustable ldifferent carrier frequencies, a pair of filter networks each capable of transmitting a band of frequencies including a different one of said carrier frequencies, means including one of said modulators responsive to said signal source and to one of said carriercurrent sources for deriving from one of said filter networks said one carrier and twor sidebands,

one of said sidebands having the desired low pass'v band width, means including said other modulator and said other filter network responsive to said frequencies derived from said one filter and said other` carrier source for deriving currents of said other carrier and two equal second sidebands each having said desired low pass band width, means for adjusting simultaneously the frequencies of said carrier current sources, and means for detecting said last mentioned equal sidebands to derive currents having substantially only frequencies within said desired low pass band.

6. A low band pass network for a source of signal frequency currents including a first modulator, a first carrier frequency current source, a

-`first band pass filter network, means for applying rcurrents including currents of frequencies to be transmitted and said first carrier currents to said first modulator to derive said first carrier and equal sideband currents, means for applying said equal sideband currents to said first filter to pass said first carrier, a complete one of said sidebands and only the desired low frequency portion of the other of said sidebands, a second carrier frequency current source, a second modulator,

having Asubstantially only lator to derive said second carrier :means for applying said second carrier currents and said first carrier and sideband currents derived from said first filter to said second modulator to derive said second carrier and second equal second sideband currents, a second band pass filter network, means for applying said second carrier and said second equal sideband currents to said second filter to derive two equal sidebands each having widths corresponding to said desired low frequency portion of said other of said first sidebands, and means for detecting said last mentioned equal sidebands to derive currents frequencies within the desired low pass band.

'7. An adjustable low band pass network for a, source of signal frequency currents including a .nrst modulator, a first adjustable carrier frequency current source, a first band pass filter network, means for applying currents including currents of frequencies to be transmitted and said first carrier ,currents to said first modulator to derive said first carrier and equal sideband currents, means for'applying said equal sideband currents to said first filter to pass said first carrier, a complete one of said sidebands and only the desired low frequency portion of the other of said sidebands, a second adjustable carrier frequency current source, a second modulator, means for applying said second carrier currents and said first carrier and sideband currents derived from said first filter to said second moduand second equal sideband currents, a secondhand pass filter network, means for applying said second carrier and second equal sideband currents to said second filter to derive two equal sidebands each having widths corresponding to said desired low frequency portion of said other of said first sidebands, means for adjusting the frequencies of said'carrier current sources, and means for detecting said last mentioned equal sidebands to derive currents having substantially only frequencies within the desired low pass band.

8. An adjustablelow band pass network for a source'of signal frequency currents including a first modulator, a first adjustable' carrier frev quency current source, a first band pass filter network, means for applying currents including currents of frequencies to be transmitted and 'said first carrier currents to said first modulator to derive said first carrier and equal sideband currents, means for applying said equal sideband currents to said first filter to pass said first carrier, a complete one of said sidebands and only the desired low frequency portion of the other of said sidebands, a second adjustable carrier frequency current source, a vsecond modulator. means for applying said second carrier currents and said first carrier and sideband currents derived from sald rst filter to said second modulator to derive said second carrier and second equal sideband currents, a second band pass filter network, means for applying said second carrier and second equal sideband currents to said second filter to derive two equal sidebands each having widths corresponding to said desired low frequency portion of said other of said first sidebands, means for adjusting simultaneously the frequencies of said carrier current sources, and means for detecting said last mentioned equal sidebands to derive currents having substantially only frequencies within the desired low pass band.

9. The method of providing low band pass transmission of signal frequency currents comprising generating first carrier currents, moda-e '.-t-lating said 'mrst carrier .currents by. applying artheretoisaidfsignal. currents, selecting apredetervi'.rn'iinedsirequency. portiomincludingfthe'carrier and eatrleastportions*ofsbothzsidebandsrof saidzmodu- `dated-carrier currents; generatngseconmcarrier `currents. of.a lfrequency.-liiflerent fthamsaid .':rst .carrier4 currents; emodulating `saidsecond carrier .gcurrents =byfapp1ying theretosaidlselectedimodu- 1 lated .nari-ier currents, .selecting fa a predetermined portion of said last mentionewmodulated carrier f @currents .and 4'detecting.zsaidfselected :portioni of fthe:last-mentioned. carrier. currents fto. derive :'Sg-

1f nal vcurrents zhaving substantially .only frequencies Within said desiredlow.passfband filmt-The method .-of. providing .adjustable lowt band .pass .transmissionvof::signal zfrequencynurl,rents comprising generatingfiirstfcarriencurrents, :emodulating asaid Afirst carrier. currents byfpply- AfingJtheretosaid signal fcurrents; selecting@ epredetermined-:frequency .portion including `rt-heicar- .4

:v1-ier and fatleast portions t of .both :sidebands of Vsaid .modulatec-fcarrier currents, generating esec- .ondscarrier currents oitaifrequency` diierent'than usaid riirstcarrier; currents, modulating said. second carrier currents byapplying .thereto saidiselected,v

modulated carrier. currents,.selecting a.predeter 'mined portion ofsaid-.lastwmentioned .carrier cur- .'rents, adjusting 'the frequencies of .said :,rst and pfaid `second carrier currents, and :detecting-@said fselected portion =of said :last mentioned..` carrier- .:currents' Ato .-derive signal .currents lhaving sub- ..stantially vonly ,frequencies .withinl said desired I dow .passband 111.*51hemethod. .of -providingadjustablezdow `t lband f pass transmissionl of signal'ffrequency .cur-` irentscomprising.generatlngbrst carrierfcurrents, 'f modulating-said :first Acarrier currentsbysaidsig- .nallcurrents, selecting a-predetermined frequency portion including the. carrierand.atrleastvportions rof -both .sidebands .of said modulated vcarrier/currents, generating second-.carrier currentsofadre- `quency different :than said :iirstncarrier currents, modulating. said second 'carrier cum-ents'by said .selected modulated lcarrier .-currentsf selecting a,

. :predetermined portion of :said last'mentioned-.curr

rents, .adjusting simultaneously the"=frequencies ofssaid rst and said secondcarrier currentseand detecting said selected :portion offsaid-ilast-amenl:tioned ycarrier currents .to .derivelsignal currents having substantially only :frequencies withingsaid-'f :desired V:low pass band.

L2. mheiimethodl offnproviding:adjustable low gmnd-I pass Li-,ransmissien ofisignal {frequency-.eur- ,z-rentszcomprisinafgeneratinglirst carrier, currents. f-.,modu1atingadrst fcarriemcurxents bassa-insig- 5 mal.:currents;selecting@predetermined:frequency mortionincluding fthe carrierlandiat leastmortions .cffbotnsidebands nffsaidmcdulated eurxentsfsen- L-.ferating secondV carrierncurrents of a :htenuency sminerent ithan-gsaid lrstvcarrier currents, modulozzlatinerfsaid l:secondmarriemcurrents :by saidfsa- .lectedmodulatedzcarrierlcurrents, rselesting;affpre :'detenninedn pontionzof said. last .mentoned..m1rfgrents, adjusting4 the -:frequency` j f `said {n1- st @earfrier; currents andfsaidxsecond 4carrier. currents. in 15 opposite senses, and detectingvsaid selected @por- :tion 1,oszaiidilast inelritionecarrier: currents .to de- Lrire-signalcurrents havingsubstantially only:;fre '-,;-quencies mithin :.said desired :low mass band.

13; The-method of :providing vadjustable:new .-ph .nd pass transmissionmf signal -frequencmcurfrrents comprising4 generating rstlcarrier currents, `.:siriodulatine:.said rstcarrier. currents by-said sig- 1 nal-,currentsfselecting-a predetennnedirequency fzportonfincluding :the carrier and. at least'ponticns 1 f Jof: bothsidebands of said'modulatedv currentsleneerating: Asecond f carrier currents :nf a Y .frequency :.different :than said rst zcarrier fcurrents, fmodulating'said second 'carriencurrents byfsaid selected modulated carriercurrents,` :selecting a predetermined portion of said .lastcmeritioned currents, -'.-adll1ting the frequency! .0f saidxfrst Icarrier. currents and said second carrierfcurrentsginf opposite :'senses andain different-:proportionateamounts.and detecting-.,.-saidselected:portion ofrsaid last-men- .,xtionedfcarrier; .currents to s derive. signal currents r havingsubstantially'lonlyfrequencies -within vsaid fidesired low rpassaband.

1 GQRDQN IL. 'FREDENDALL.

:BEEERENCES :CITfED UNITED, :STATES .PATENTS 4are of -record in the 45, iNumber Nam-e Date :1,861,488 Osborne Dec. 7, A19120 11;830j896 Wintringham gNov.` 10; i931 Lf1,905;714 Horton ;.Aprli25g r1933 w .'l-=;2;339,'633 Gilman .Ian2'18,*1944 

